Yes. But, if you do this sort of piece-work, you would have usually signed a service agreement which specifies that you only provide services on demand with advanced booking and at an hourly or daily rate of X. If that's the case, the employer shouldn't really be in a position to restrict your access to jobs with other parties in the rest of the time. It's a completely different matter if you're in a full-time, salaried position though. Then, you recognise that whoever pays your salary is your primary employer and you may not be allowed to work elsewhere on the side without prior written permission. Many air operators tend to be quite strict with the aforementioned as they're responsible for compliance with FTL and keeping your flight and duty time records.